Currently the means of obtaining carriage positional feedback in a two-axis machine such as a lathe, has involved fitting a linear position sensing device between each moving axis and the base of the machine. Thus the relationship of position between a tool and a workpiece is derived from two sets of feedback information, namely
1. a point on the workpiece carriage which is monitored for change of position with respect to the base, and PA1 2. a point on the tool carriage which is monitored for change in position with respect to the base. PA1 A) variation in relationship between the machining point and the reference point of the work-feed measuring system, PA1 B) similar variations in the relationship between the machining point and the tool feed measuring system, and PA1 C) any static or dynamic changes in the relationship between the two points on the base to which the two axis measuring systems are attached. PA1 1. orthogonality of the axes of the tool carriage and workpiece carriage, PA1 2. errors contributed by the machine base, including static low frequency dynamic compliance, geometric stability, thermal expansions and distortions (all of which can affect the angle of one of the said axes relative to the other), and PA1 3. lateral axial and angular deviations and disturbances of the guideways. PA1 (A) distortions, deflections, expansions etc, occurring in the machine mechanics situated in the direct path between the cutting tool and grating reference point, and between the workpiece cutting zone and the direct path to the grating reference point. PA1 (B) errors in the grating itself. These errors are not necessarily trivial. Scaling errors, orthogonality of the X and Z fringes, interpolation errors and any other errors in the grating will be directly applied to the workpiece.
However the position on the base which is checked against the point on the tool carriage is not the same as the point on the base which is used as the reference for monitoring the position of the point on the workpiece carriage. As a consequence errors can arise in the computation of the position of the tool relative to the workpiece at the point of engagement due to the following possibilities:
It is an object of the present invention to provide an improved method of measuring the position of workpiece carriage and tool carriage so as to reduce the error in the computation of the tool/workpiece point of engagement and to provide a machine capable of high precision machining.
It is also an object of the invention to provide a design of machine which does not require the same degree of build accuracy as is necessary using conventional carriage monitoring systems and should therefore be considerably cheaper to build.
Although the invention may be applied to machine tools in which the workpiece carriage and/or the tool carriage move through considerable distances during workpiece machining, the invention is in fact of primary application to machine tools having a small working volume, in which both the workpiece carriage and the tool carriage only move through relatively small distances during the machining operation.